Limited POSEIDON
Ernesto Carrella
8/4/2020
Intro
Poseidon in Indonesia
Approach:
Take catch-at-length data
Feed it into length-based assessment techniques
Estimate population/carrying capacity
Run realistic POSEIDON over it
Advantages:
Best available science
Realistic
Can focus on details
Disadvantage
Opaque:
How data affects model
What is important
Slow:
Changes in assumptions/data require starting from scratch
Uncertainty poorly defined
A possible workaround
Keep your model simple
Isolate parameter/state space by rejection sampling
Focus less on:
The “right” parameters
The “right” distribution of parameters
Focus more on:
a large ensemble of valid histories
How rejection sampling works
Run the model “many” times
Start filtering
Keep filtering (SPR)
Keep filtering (Landings 2)
Keep filtering (SPR 2)
Simplified 712
Simplified model
2 species (Lutjanus malabaricus + Pristipomoides multidens)
10x10 simple map
2 fleets (0-9; 10+)
Things we don’t know:
Initial population
Carrying capacity
Current depletion and the like
Things we know:
Last year landings
SPR
data we collected
Approach:
Start the fishery “ab novo”
Repeatedly:
Randomize all parameters
\(K,L_{\inf},\dots\)
Run simplified POSEIDON for “many” years (up to 40)
Free entry
Accept simulation if model output are the same as what we observe in data
Collect
Why bother?
Benefits:
No need for population reconstruction
Clear uncertainty propagation
Clear value of information
Cost:
Model needs to be fast
Results
Ran the model for about 50,000 times
Accepted about 1.5% of them
This is their story….
Weighing evidence
Landings only scenario
Imagine we only knew what this year landings (no length data)
5000t to 10000t for malabaricus
1000t to 3000t for multidens
Knowing SPR
How does it change if I also add as a condition
OBSERVED SPR of malabaricus must be between 0.2% and 20% (measures from 2018 and 2016 but adding a bit of pessimism) respectively
Knowing SPR of other species
How does knowing that the SPR of the other species is between 10 and 20 help?
Landings only (2)
Knowing SPR (2)
Knowing SPR of other species (2)
Basic results
Once SPR is properly contextualized within the history of the fishery; these are actually green shoots
Measuring the SPR of another species can increase our expected estimate of depletion for this species
Additional knowledge
I know that both boat populations are still at it
I know that small boats are more numerous
I know that malabaricus SPR has dropped dramatically?
Additional Knowledge 2
Additional Knowledge 3
Policy
Policy attempts
We don’t want just to assess, we want to know policy effects:
no policy
put a 100 day limit to 10GT only
put a 100 day limit to all
Data-limited
Works as before but:
apply same policy to all accepted runs
Positive:
study the average effect
understanding the uncertainty
Negative:
to make them comparable, speak only in relative terms
Results 1
Results Biomass
Results Landings
Results Cashflow
718
3 Species
Lutjanus malabaricus
Atrobucca brevis
Lethrinus laticaudis
3 Fleets
Large Longliners
Large Dropliners
Large Gillnetters
718 - Posteriors
718 - Policy
718 What’s missing
Price increase
Keep probolinggo boats very far
Deal with seasonality of dropliners